Water soluble quaternary cationic dyes of the formula ##SPC2##
Wherein R.sub.1 and R.sub.2 have the meaning assigned above, X.sub.3 is alkyl and A is an anion, are known.
These dyes have heretofore been prepared by the quaternization of a compound of the formula (I) with an excess of a dialkyl sulfate in an inert organic solvent such as chlorobenzene, chloroform, benzene, xylene, acetonitrile or the like. This procedure, however, has disadvantages in commercial practice since the use of a solvent presents an undesirable hazard and since an additional step of solvent recovery must be carried out in order to make the procedure economically practical. Further, in order to bring the reaction to completion, it is necessary that the reaction be carried out at elevated temperatures, particularly at or near the boiling point of the solvent.
The use of such high temperatures results in the formation of tarry and oily impurities, thus making it necessary to clarify the solvent-free cake by dissolving it in water and treating the solution formed with a filter aid such as charcoal before isolation of the cationic dye.
A further disadvantage of the use of organic solvent in the quaternization of compounds of formula (I), is that even when the reaction is carried out at the boiling point of the solvent unreacted material remains which is difficult to separate from the product and accordingly the dyestuffs obtained give a reddish staining on wool and cotton which is not in tone with the blue dyeing obtained on acrylic fiber. This is a definite disadvantage in the dyeing of fiber blends. Dyestuffs prepared by the process of the present invention either do not stain on wool or cotton or give slight stains which are in tone with the blue coloring on acrylic fibers.
It is desirable to provide an economical, commercially practicable process for the preparation of quaternized benzthiazole-azo dyestuffs.